Conventional tractors used in agricultural operations are adapted to pull various ground cultivating implements over the field to be cultivated. Typically, tractors have two means of attaching such implements to the tractor. Most tractors have a simple drawbar which is normally centrally located at the rear of the tractor and which functions in the manner of a simple trailer hitch. In addition, most tractors also have a three-point hitch, also mounted at the rear of the tractor. A three-point hitch typically includes two points of attachment located on lifting arms spaced on either side of the center line of the tractor. Implements attach to them in pinned relation, so that the implements may be raised or lowered relative to the tractor by raising or lowering the lifting arms driven by the tractor hydraulic system. A third point of attachment is located above the other points of attachment, and midway between them. The third point of attachment comprises a link which is pinned to the tractor at one end and to the implement at the other end to complete a stable, "three-point" connection between the tractor and implement. Thus, by hydraulically lifting the lifting arms, the cultivating implement and implement end of the center link are lifted to position the implement or raise it off the ground for turning or transporting. Conversely, lowering of the lifting arm positions the implement for tillage or other purposes.
An advantage of the three-point hitch system is that the weight of the implement may be transferred to the rear wheels of the tractor to increase traction during use of the implement. A disadvantage of the three-point hitch system is that the implement, when elevated, exerts a downward torque effect on the rear of tractor which tends to lift the front end of the tractor, with the tractor rotating about its back wheels. This effect is increased with the weight of the implement and its distance from the rear wheels. As a consequence, a three-point hitch system on a particular tractor is subject to certain limits of implement weight and rearward extension.
It has become increasingly economical to cultivate large fields in such a way as to require the fewest possible trips with the tractor over the field. Thus, instead of covering the field first with a primary cultivating implement, such as an array of disks, and then with a finishing implement, such as a rotary harrow, it is advantageous to be able to mount the two implements one behind the other. Conventionally this is done by combining them into a single, compound machine. Examples include the cultivating implements shown in van der Lely, U.S. Pat. Nos. 3,937,460; van der Lely, 3,983,943; van der Lely, et al., 4,057,111; Dail, Jr., et al., 4,088,083; and Weichel, 4,324,295. Each of the implements shown in the patents referred to is adapted to be attached to a three-point hitch. The implements are lifted as a unit by means of the three-point hitch when it is desired to disengage them from the ground. As a consequence, when in the raised position, a great deal of torque is applied to the tractor, effectively limiting the size of the compound implement that may be achieved.
Another characteristic of a three-point hitch mounted implement is that the connected rigid implement remains generally aligned with the center line of the tractor during use. As a consequence, a cultivating implement so mounted is moved transversely through the soil if it is not lifted from the ground when the tractor turns. This effect is increasingly important as the implement is made either wider or longer. Consequently, when it is desirable to leave a wide or long implement in contact with the ground during turns, or when an implement is so heavy that to raise it with a three-point hitch threatens the stability of the tractor, it is conventional to attach the implement to the tractor by the tractor's drawbar. By this means, the implement can freely trail the center line of a tractor in a turn, following the tractor has would any trailer.
Not using the three-point hitch requires some alternative means of lifting the cultivating parts of the implement above the ground. Conventionally, wheels are provided for the implement, which wheels may be moved from a raised position, in which the ground cultivating parts of the implement are in contact with the ground, to a lowered position, in which the wheels bear the weight of the implement and hold its cultivating parts above the ground. An example is the pull-type rotary harrow sold by the Lely Corporation under the trademark Lely-Roterra.RTM. RH 900-60. More than one such cultivating implement can be attached as a train drawn by a single tractor to be controlled independently of each other, each with its own means for raising and lowering the cultivating parts of the implement. The result is a complicated and cumbersome arrangement.